Windings for electromagnets



Aug. 19, 1958 LQR. NIXCN WINDINGS FOR ELECTROMAGNETS Filed March 25,1955 I unu- United States Patent WINDINGS FOR ELECTROMAGNETS LeslieReginald Nixon, deceased, late of Catford, London, England, by EdithMaud Nixon, executrix, Catford, London, England, assignor to J. Stone &Company (Deptford) Limited,Londou, England This invention concernsexciting windings for electromagnets, particularly such for automaticregulators,'relays and the like, and has for its object to .providebysimple means effective compensation for variation of the resistance ofsuch windings with variation of temperature.

According to the invention, an electromagnet comprises, in addition to amain Winding, an auxiliary winding connected in one diagonal of a bridgecircuit across whose other diagonal the source of supply of current isto be connected and whose pairs of opposite limbs comprise two parts ofa divided main winding, or the main winding and a resistance, and twoballast resistances respectively, the bridge circuit being arranged tobe balanced at a normal mean temperature and, on unbalance due totemperature change, to produce in the auxiliary winding a magneto-motiveforce opposite to and substantially equal to the variation, produced bythe temperature change, in the magneto motive force of the main winding.Thus the magneto-motive force of the auxiliary winding can alternativelyoppose or assist that of the main winding so as to provide an effectivemagnetomotive force which remains substantially constant over a range oftemperature change.

Various ways of carrying the invention into effect will now be describedby way of example and with reference to the accompanying drawing inwhich:

Figures 1 to 3 are circuit diagrams showing different arrangements.

Fig. 4 is a diagram illustrating the relative disposition on the magnetof the windings shown in the circuit diagram of Fig. 1.

In all of the arrangements illustrated, the magnet is provided with amain winding divided into two equal parts .9, and s and with anauxiliary winding a. The latter is connected in one diagonal of a bridgecircuit of which one pair of opposite limbs comprises the two parts sand s of the main winding and the other pair comprises two equal ballastresistances l l The other diagonal of the bridge is connected to thesource b of current. The ballast resistances are made substantiallyinsensitive to temperature variation. The magnet windings are preferablyall of copper.

The parts s and s of the main Winding are arranged to assist each other.The auxiliary winding a may be unexcited or may assist or oppose themain winding depending upon conditions in the bridge. The resistances ofthe limbs s s and l 1 are designed so that the bridge will be balancedand the auxiliary winding unexcited approximately at the normal meantemperature in the working range of the electromagnet. If the resistanceof the winding parts s s rises, so that the magneto-motive force of themain winding decreases, due to increase of temperature above the valueat which the bridge is balanced, a current will flow in one direction inthe auxiliary winding a. The winding is so arranged on the magnet thatcurrent in this direction will produce a magneto-motive force whichboosts the magnetomotive force of the main winding. The former magneto-2,848,656 Patented Aug. 19,1958

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motive force can be made substantially to compensate for the decrease inthe magneto-motive force in the main winding. If the resistance of theWinding parts s s falls, due to decrease of temperature, therewill be acurrent in the opposite direction through the winding a and amagneto-motive force opposing thatof the main winding. In either case,the effective magneto-motive force will remain substantially unaltered.

The relative directions of the. aforesaid magnetizing forces produced bythe currents in the windings are indicated inFig. 4. t i

As illustrated in Figure 2, a further ballast resistance 1 may, ifdesired, be connected in series with the whole bridge. r

If the windings s s and a require to beaexcited' by direct currentandthe source of supply. is'alternating,

current, a rectifier may be providedui An arrangement of this kind isshown in Figure 3 in .Whicha;full-wave rectifier c is interposed betweenthe source b and the bridge circuit described with reference toFigure 1. The additional ballast resistance 1 may be provided in thiscase also, as indicated in broken lines.

If the same compensation is required for variation of windingtemperature due to self heating as for that due to ambient changes, itmay be advantageous to interleave the main and auxiliary windings. Onthe other hand, if desired, compensation for the two effects may be madeunequal by arranging for unequal temperature rises in the windings dueto self heating.

Such arrangements can be advantageously applied to electro-magneticregulators, for example to the carbonpile regulator described in thespecification of the patent application No. 399,974. In such anapplication, the said arangements have certain advantages overcompensating arrangements utilising, for example, a bimetal ele mentacting on the control spring of the regulator: For one thing, thecompensation is independent of the setting of the control spring.Moreover, as the net magnetomotive force is substantially unaitected bytemperature, undesirable effects due to changes in the force/deflectioncharacteristic of the magnet system are obviated: For instance, a changein the cut-in voltage with temperature is avoided. Transitory errors due'to sudden changes of ambient temperature can also be avoided, as thethermal time constants of the two windings can be made similar. This isof particular benefit in applications used on aircraft.

For some purposes, instead of dividing the main winding of theelectromagnet between two limbs of the bridge circuit, the undividedmain winding may be arranged in one limb and an equivalent resistance inthe opposite limb of the said circuit, whereby a similartemperaturecompensation effect can be obtained.

However, the invention is applicable with similar advantages to magnetwindings of apparatus other than regulators, for instance relayssaturable chokes, motor and generator fields, and generally, to anywinding whose magneto-motive force requires to betemperature-compensated.

What is claimed is:

1. An electromagnet comprising a magnet and, in addition to a mainwinding divided into two parts disposed on said magnet and, an auxiliarywinding disposed on said magnet and connected in one diagonal of abridge circuit across Whose other diagonal the source of supply ofcurrent is to be connected and Whose pairs of 0pposite limbs comprisethe two parts of the main winding and two ballast resistancesrespectively,. the bridge circuit being arranged to be balanced at anormal mean temperature and, on unbalance due to temperature change, toproduce in the auxiliary winding a magnetomotive force opposite to andsubstantially equal to the variation, produced by- 'the' t'emperaturechange, in the magneto-motive force on the main winding.

2. An electromagnet comprising a magnet and in addition to a mainwinding disposed on said magnet, an

auxiliary winding, disposed bni sa'id 'm'agn'et connected in onediagonal of a bridge-"circuit across'whose' other diagonal the-source ofsupply of current is to be connected and whose'pairs of opposite limbscomprise the balance due tortemperaturechange, to produce in theauxiliarywinding a magneto-motive force opposite to and substantiallyequal to'the variation, produced by the temperature change, in themagneto-m-otiveforce of the main winding. H

I 3. An ele'ctromagnet as elaimedin claim 1, wherein a further ballastresistance is connected in series with the 1 bridge circuit.

4. An electromagnet as claimed in claim L -wherein a rectifier isinterposed between'the source of supply and the bridge circuit.

S. electromagnetic carbon-pile regulator provided "with atemperature-compensated electromagnet comprising a magnet and, inaddition to a main winding divided into two parts disposed on saidmagnet, an auxiliary winding disposed on said magnet connected in onediagonal of a bridge circuiva'crbss Whose other diagonal the source ofsupply of current is to be connected and whose pairs ofoppositelimbscomprise the two parts of the main winding and two ballast resistancesrespectively, the bridge-circuit being arranged to' be balanced at a'normal'mean temper'ature'and, on unbalance due to temperature change,to produce in' the-auxiliary winding a magneto-motive force opposite toand substantially equal to the variation, produced by the temperaturechange, in the "magneto-'motive fo'rce of the main winding.-

References Cited in the file of this patent UNITED STATES PATENTS

